There is interest in methods for the synthesis of large numbers of diverse compounds that can be screened for various possible physiological or other activities. Techniques have been developed in which one adds individual units sequentially as part of the chemical synthesis to produce all or a substantial number of the possible compounds which can result from all the different choices possible at each sequential stage of the synthesis. WO 93/06121, Apr. 1, 1993, teaches methods for solid support-based synthesis of random oligomers wherein identification tags on the solid supports are used to facilitate identification of the oligomer sequence synthesized. A detachable tagging system is described in Still et al., WO 94/08051, Apr. 14, 1994, which teaches the synthesis of large combinatorial libraries of compounds attached to solid supports.
In order to screen the compounds produced by these new synthetic methods, it is desirable to partition from a pool containing very large (up to the range 10.sup.6 -10.sup.9) numbers of solid supports into collection plates with wells, typically 96 wells containing 1-30 supports per well. These solid supports, commercially available as beads, are generally 50-1000 .mu.m in diameter. The number of beads per well is a matter of choice, but should be at least one and not greater than 200, constrained by screening statistics, solubility factors, or size. Furthermore, the number per well should be consistent. Therefore, no well should be empty. However, an average variation of .+-.5 beads per well is acceptable for wells containing 20 beads. When single beads are to be screened in each well, it is desirable to minimize empty wells while avoiding multiple beads per well in order to avoid the need for rescreening these beads.
Various devices are known in the art which sort particles from liquid suspensions. For example, U.S. Pat. No. 3,710,933, Fulwyler et al., describes a particle sorter applicable to the sorting of biological cells which analyzes cells in a flow chamber and then produces cell-containing droplets via a piezoelectric crystal. Emerging droplets are sorted into two receptacles. Droplets containing selected cells are electronically charged and then deflected by a static electric field into one receptacle. Unselected cells drop into the other receptacle. U.S. Pat. No. 4,173,415, Wyatt, describes an apparatus for characterizing organic cells such as leukocytes which creates an aerosol from a cell suspension to produce a series of droplets which are then divided into cell-containing and non-cell-containing streams. The cell-containing stream is then air-dried and the cells finally analyzed by monochromatic light. U.S. Pat. No. 4,606,631, Anno et al., describes a particle counter which utilizes a flowing sheath solution to surround the sample solution which contains the particles, typically blood corpuscles, to be counted. U.S. Pat. No. 4,680,977, Conero et al., teaches an apparatus for sensing the flow of a liquid by detecting and measuring drops through an optical drip chamber. U.S. Pat. No. 4,655,265, Duteurtre et al., describes an apparatus for the batch transfer of brittle particles, specifically yeast-containing alginate beads, from a suspension into fermentation containers. U.S. Pat. No. 5,142,140, Yamazaki et al., describes an apparatus which uses a polarized beam splitter for counting particles, typically leukocytes, suspended in a fluid. U.S. Pat. No. 5,166,537, Horiuchi et al., describes an improved Coulter Counter device which utilizes a light detection method in combination with an electric impedance method and compares the signals therefrom to exclude false data obtained from multiple particles being present simultaneously. U.S. Pat. No. 5,286,452, Hansen, teaches a method for analyzing multiple analytes in a single fluid sample and a sheath-type flow cell for performing said method. None of these methods and teachings are suitable for accurately counting a number of small objects, including beads in the size range mentioned above removed from a fluid reservoir, and depositing all or a pre-selected number of them into one or multiple containers in the form of droplets.